The present invention relates to a method for preparing semipermeable membrane compositions for the separation of components from fluid mixtures or solutions.
Separation processes such as dialysis and reverse osmosis have been used in the separation of a wide variety of impurities and components from solutions. Membranes have been developed and used, for example, in the desalination of brackish and saline waters. Membranes have also been developed for the separation of gases. Membranes have been used for the separation of acid gases from methane, as described in U.S. Pat. Nos. 3,534,528; 4,130,403; and 4,561,654, or the separation of other gas mixtures, as described in U.S. Pat. No. 4,230,463.
The essential characteristics of semipermeable membranes can generally be defined in terms of relative permeability to the specific components to be separated, permeation flux for one of the components to be separated and mechanical strength of the membrane composition. The permeation flux is a measure of the rate at which one of the components to be separated permeates through the membrane. The relative permeability of the membrane is the ratio of the permeation fluxes of the components to be separated, often referred to as the separation factor or selectivity of the membrane.
A conventional technique for preparing semipermeable membrane compositions involves casting a polymeric solution to form a film on a surface and then completely evaporating the solvent. This method can produce semipermeable membranes which possess good physical strength and membrane characteristics, but the thickness of such homogeneous membranes causes an undesirably low permeation flux. Because the permeation flux is generally inversely proportional to the thickness of the membrane device, the membrane is advantageously as thin as possible. Unfortunately, extremely thin membrane structures prepared by conventional casting methods lack the desired selectivity and mechanical strength. They frequently contain discontinuities, small holes or other defects.
To correct the foregoing deficiencies, membrane compositions having a microporous structure for support and a dense layer for separation have been prepared by the so-called Loeb-Sourirajan method. This method produces asymmetric membranes which possess good physical strength and membrane characteristics. Known membrane compositions include cellulose ester membranes, which are typically employed in flat sheet or hollow fiber form. Unfortunately, the method involves process limitations which are difficult to control and, as noted in U.S. Pat. No. 4,430,807, can require special drying techniques if used for gas separation.
There have been numerous attempts to improve the membrane compositions and the processes for their preparation. Henis et al., Science, Vol. 20, page 11, 1983, disclose coating a gas separation membrane with silicone rubber in an attempt to plug flaws and defects present in the membrane. Unfortunately, such coated membranes normally exhibit permeation selectivities intermediate between that of the silicone rubber and the uncoated membrane. Delamination of the coating can also occur in the presence of condensed liquid hydrocarbons, as may occur under certain operating conditions.
In view of the deficiencies of the prior art, it would be highly desirable to provide a method for preparing semipermeable membrane compositions that exhibit increased permeation flux without sacrificing selectivity or mechanical strength.